1. Field of the Invention
The present invention relates to an ink-jet head for use in a printer or the like, and particularly to an ink-jet head, in which an ink chamber is defined by walls containing a piezoelectric member, and a voltage is applied to the piezoelectric member to cause deformation and thereby to cause pressure vibrations in the ink chamber so that an ink accumulated in the ink chamber is ejected therefrom.
2. Description of the Related Art
In recent years, non-impact printing devices, e.g., of an ink-jet type, which can be easily configured to perform color printing and multilevel gradation, have been rapidly spreading instead of impact printing devices. As an ink-jet head used as an ink ejecting unit in the non-impact printing device, attention has been given particularly to a drop-on-demand type, which can eject only ink droplets required for the printing, in view of high ejection efficiency and easy reduction of cost. A Kyser type and a thermal jet type are mainstream of the drop-on-demand type.
However, the Kyser type does not allow easy reduction in size, and thus is not suitable for a high-density structure. The thermal jet type is suitable for a high-density structure, but requires ink having appropriate heat resistance because it uses a heater for heating the ink to generate bubbles, and ejects the ink by using energy of the bubbles. Also, it is difficult to provide a long-life heater. Further, an energy efficiency is low, and therefore power consumption is large.
An ink-jet type utilizing a shear mode of a piezoelectric material has been disclosed as a type overcoming problems of the foregoing types. In the ink-jet type, electrodes are formed on opposite side surfaces of a wall (which will be referred to as a “channel wall” hereinafter) of an ink channel made of a piezoelectric material, and an electric field perpendicular to a direction of polarization of the piezoelectric material is produced by using these electrodes so that the channel wall is deformed in a shear mode, and pressure wave variations caused thereby are utilized to eject ink droplets. This type is suitable to high-density arrangement of nozzles, low power consumption and high drive frequency.
Referring to FIG. 14, description will now be given on a structure of an ink-jet head utilizing the shear mode. This ink-jet head includes a base member 1, which is made of a piezoelectric material subjected to polarization processing in a vertical direction of FIG. 14, and is provided with a plurality of channel grooves 4, a cover member 2 provided with an ink supply port 21 and a manifold space 24, and a nozzle plate 9 having nozzle holes or orifices 10. Base member 1, cover member 2 and nozzle plate 9 are fixed together to form an ink channel. The “ink channel” is a portion of a pressure chamber formed by utilizing an inner space of channel grooves 4. On only an upper half of a channel wall 3, electrodes 5 are formed for applying an electric field. In the following description, the side, on which nozzle plate 9 is present, will be referred to as the front side, and the opposite side will be referred to as the rear side. In this ink channel, the rear end portion of each channel groove 4 is worked into a round or curved form corresponding to a diameter of a dicing blade used for forming the groove. Further, a shallow groove portion 6 is also formed by the dicing blade to provide an electrode leading portion for external electrical connection. The electrode formed in shallow groove portion 6 is connected at its rear end, e.g., to an external electrode 8 of a flexible printed board 11 by a bonding wire 7. In the ink-jet head of the above structure, the ink is supplied from manifold space 24 through the region of the round form. However, the pressure required for the ejection is to be generated in the region, where the upper portions of channel walls 3 of base member 1 are joined and fixed to cover member 2. Thus, the region of the round form is not required for such pressure generation, and becomes a cause of increase in electrostatic capacitance.
Japanese Patent Laying-Open No. 9-94954 has disclosed a structure of the ink-jet head, which is not provided with a round-shaped region and thereby can reduce the electrostatic capacitance. In the ink-jet head disclosed in this publication, however, connection is performed on a bottom surface of a base board for externally pulling out an electrode from a channel wall so that complicated steps are required for forming the electrode for connection.
As a structure, which can reduce an electrostatic capacitance of an ink-jet head and allows easy pull-out of an electrode from a channel wall, a structure shown in FIGS. 15 and 16 is already disclosed. FIG. 15 is a perspective exploded view of the ink-jet head with certain parts cut away. FIG. 16 is a cross section of an assembly of the ink-jet head. This ink-jet head has a feature that channel grooves 4 extend longitudinally throughout base member 1 with uniform depth. This structure can eliminate a round-shaped region, and can reduce an electrostatic capacitance. Also, it is possible to reduce a used amount of a piezoelectric material. Channel grooves 4 are sealed in the vicinity of their rear ends by electrically conductive resin 26 so that electrical connection is established to keep the same potential at electrodes 5 on the opposite sides of each channel groove 4. Conductive resin 26 reaches the rear ends of channel grooves 4, and the rear end of base member 1 is connected to flexible printed board 11 with anisotropic conductive film (which will be referred to as an “ACF” hereinafter) 12 therebetween. External electrode 8 on the surface of flexible printed board 11 and conductive resin 26 are pressed together in a direction of thickness of ACF 12 interposed therebetween, and thereby are electrically connected together. However, each ink channel is electrically independent of the others owing to characteristics of ACF 12.
In a conventional ink-jet head, conductive resin 26 in a liquid state is applied to the vicinity of the rear end of channel grooves 4, and then is cured. Therefore, a crack may occur between conductive resin 26 and channel wall 3 due to cure shrinkage of conductive resin 26. Conductive resin 26 is cured at a raised temperature, but conductive resin 26 has a larger coefficient of linear expansion than the piezoelectric material of base member 1. Therefore, thermal shrinkage, which occurs due to cooling after the curing, may cause a crack between conductive resin 26 and channel wall 3. FIG. 17 shows an example of a crack caused by the above reason. A crack 16 is present between conductive resin 26 and channel wall 3. This crack may cause a failure in electrical connection between electrode 5 and external electrode 8.